The present invention relates to a catalyst bed system for an endothermic dehydrogenation process and an endothermic dehydrogenation process using said catalyst bed system.
The invention is concerned with an endothermic hydrocarbon process, particularly for adiabatic catalytic dehydrogenation of paraffinic and other hydrocarbons such as propane dehydrogenation (reaction 1) or butane dehydrogenation (reaction 2):C3H8 C3H6+H2  (1)C4H10 C4H6+2H2  (2)
Dehydrogenation of hydrocarbons, in particular aliphatic hydrocarbons, to convert them into their respective olefins is a well known process. For example, the hydrocarbons propane, butane, isobutane, ethyl benzene are well known and catalytically dehydrogenated to produce the respective propylene, butene, isobutene, butadiene and styrene. Dehydrogenation reactions are strongly endothermic and thus, an increase of the heat supply favours the olefin conversion.
One well known dehydrogenation process is the Houdry CATOFIN® process in which an aliphatic hydrocarbon is passed through a dehydrogenation catalyst bed where the hydrocarbon is dehydrogenated to the respective olefin, the olefin is flushed from the bed, the catalyst is regenerated and reduced, and the cycle is repeated (U.S. Pat. No. 2,419,997).
Another well known process is the CATADIENE® process in which butanes and butenes are dehydrogenated to produce butadiene. This process is again a cyclic process, similar to the previous mentioned CATOFIN® process.
Typically the catalyst is heated by contact with a heated gas, usually air. The aliphatic hydrocarbon such as propane is passed through the hot catalyst bed which supplies the heat for the dehydrogenation reaction. Since the dehydrogenation reaction is endothermic and the process is adiabatic, the temperature of the catalyst gradually drops during the dehydrogenation cycle and causes a decrease in hydrocarbon conversion rate. Particularly the temperature at the top of the catalyst bed decreases by as much as 100° C. (US 2007/0054801 A1).
In order to reheat the catalyst bed and remove coke that has deposited on the catalyst during the dehydrogenation step, the reactor is purged of hydrocarbon and undergoes a regeneration step with air heated to temperatures up to 700° C. Heat is provided to the bed by the hot air that passes through the bed and also by the combustion of coke deposits on the catalyst. After reheating and regenerating the catalyst and before putting the reactor back on stream, the oxidized catalyst must be reduced by passing a reduction gas such as hydrogen or hydrocarbon through the catalyst bed. This also supplies additional heat by the oxidation of the reducing gas (US 2007/0054801 A1).
The question of additional heat supply to the reactor bed in order to compensate for the heat consumption during the strong endothermic reaction has been addressed in the past and different approaches have been suggested.
For instances U.S. Pat. No. 6,392,113 B1 proposes to react the hydrocarbon feed partially in a catalytic pre-reactor and reheating the exiting hydrocarbon stream from pre-reactor to the same preheat temperature prior to introduction into the main catalytic reactor. Reheating can be done either by exiting regeneration air or by a separately fired heater.
WO2007/030298 A1 describes an improved catalyst bed system. In one part of the catalyst bed, the propane dehydrogenation catalyst is mixed with any inert material which does not produce heat during the whole process and in another part of the bed, propane dehydrogenation catalyst along with normal inert material, a new material which is catalytically inert with respect to dehydrogenation, cracking, coking reactions and generates heat when it is exposed to reducing and/or oxidizing reaction conditions.
US2006/0004241 A1 discloses a way to decrease the temperature gradient in isothermal propane dehydrogenation process. This patent proposes to dilute the catalyst bed with inert particles at the beginning of the catalyst bed to decrease the temperature gradient in the section of catalyst bed for the isothermal propane dehydrogenation process.
WO2007/018982 A1 uses two different catalyst zones for adiabatic, non-oxidative hydrocarbon dehydrogenation processes. The first zone of catalyst has higher activity and higher capacity to produce more coke than the second layer. The combustion of coke in turn provides extra heat.
A similar approach is applied by U.S. Pat. No. 4,560,824 A, which proposes additional heat by increasing coke production. The coke production is caused by adding unsaturated components to the feed during dehydrogenation.
As can be seen from the prior art there are several possibilities to increase the heat during the dehydrogenation process, in particular by increasing the hydrocarbon feed temperature, increasing air inlet temperature and increasing air flow rate. Additional heat can also be provided by injecting and combusting a fuel gas with regeneration air and increasing flow rate of said injection gas.
However, there are several challenges associated with the above described possibilities.
An increase of temperature of the hydrocarbon feed could create cracking of hydrocarbons to lighter products and coking in the pipelines. An increase of the air inlet temperature or injection gas flow rate in order to increase the temperature in the catalyst layer could accelerate catalyst deactivation. Furthermore, once designed, a plant runs to the limit of air handling equipment. Increasing air flow requires major capital expenditures (U.S. Pat. No. 6,392,113 B1).